CD59 (protectin, membrane inhibitor of reactive lysis [MIRL]) is a glycan- phosphatidylinositol (GPI) linked protein that is crucial in the control of complement activation by inhibiting the polymerization of C9. It is particularly important for cells of the blood and its lack in blood cells in paroxysmal nocturnal hemoglobinuria is the chief cause of hemolysis and perhaps thrombosis. Using a cDNA for CD59, we plan to characterize the gene structure of the molecule. We plan to determine structure-function relationships by replacing the GPI anchor with a transmembrane anchor and to vary the length of the cytoplasmic tail in order to determine the effect on its biological function. Using site-directed mutagenesis, we plan to determine which of the 11 cysteines in the molecule are important for function. We will replace segments of the molecule to determine the functional sites. We will examine the effect of altering the carboxyl end of the coded molecule on the fixation to the GPI anchor; specifically, we will mutagenize the three asparagine groups as well as specific residues within the hydrophobic region of the molecule. Finally, we will exchange this region with that of CD55 to determine if the hydrophobic region is protein specific. With these studies, we will have an increased understanding of the genetics and structure-function relationships of this important protein.